Use better naming for swap copy/erase functions

Since a copy/erase during a swap can result in some number of sectors
erased and copied, which is not necessarily equal to 1, update design
document and bootloader function naming to reflect this fact, by using
the postfix "_region" instead of "_sector" in those cases.

This fixes #541

Signed-off-by: Fabio Utzig <utzig@apache.org>